The moisture content (MC) of wood is the amount of water contained in the wood and includes the water absorbed into the wood cell walls and free water within the hollow center of the cell (the MC is expressed in percentage in weight). Most species of wood can absorb around their cell wall a maximum of 25 to 30% of water. This limit is called the fibre saturation point. That saturation point can be reached by absorbing liquid water (in exposing the wood to rain for example) or by absorbing water vapour (in exposing the wood to air having a high level of Relative Humidity (RH)). The only way to bring the MC of wood above its fibre saturation point is with excessive exposition to liquid water only coming from windblown rain, leaks, condensation, melting ice or snow, etc. When all the air in the hollow center of the cell is replaced by water, the wood is waterlogged and the MC can be as high as 200%.
Below the fibre saturation point, the amounts of water vapour which can be absorbed by wood depend on the RH and the temperature of the air. If a piece of wood is exposed for a long period of time to an environment where the air is at 70° F. with a RH of 20%, the MC of the wood will eventually reach 5.4%. If the wood is exposed to an environment condition where the temperature is 90° F. with a RH of 90%, the MC of the wood will eventually reach 19.8%. When the MC of the wood is balanced with the RH and the temperature level, it is said that the wood is at its equilibrium moisture content (EMC). This rarely happens in nature because the RH and the temperature of the environment are consistently changing, and so does the MC of the wood. It is known that the MC of wood which is exposed to outside conditions in the United State will stay between 10% to 18.5% (except for some dry areas such as in the states of Arizona, Nevada or Texas, where the MC can go as low as 4%).
Weathering is the general term used to describe the degradation of hardwood (or softwood) exposed to outside conditions (where MC of the wood will vary under the fibre saturation point). The process of degradation of the wood is activated by sunlight radiation, temperature changes, washing by rain, and repetitive change in moisture content of the wood. This degradation occurs mainly on the surface of the wood. Swelling and shrinking stresses created by MC variations will accelerate the deterioration of the surface of the wood. Repetitive exposure of the wood to a pattern of wetting and drying causes differentiation of the wood itself and will result in many small or larger checks and cracks. Also, wood components that are warmed by the sunlight will become drier. The top surface will become drier than the rest of the board creating checks and cracks parallel to the grain of the wood.
Decay can occur only when the MC of the wood fibre is above the fibre saturation point, where fungi can develop. Wood kept consistently dry does not decay. (For more detail about physical properties of the wood, the moisture content of wood or weathering and wood decay, see “Wood Handbook: Wood as an Engineering Material”, General Technical report FPL-GTR-113, United States Department of Agriculture).
Thus, it is well known in the wood industry that when the wood is exposed to outside conditions, to prevent its deterioration and increase its durability, it is necessary to control the variation of its MC. Reducing the variation of the MC of the wood will diminish the weathering effect and maintaining the MC variation of the wood under its saturation point will prevent wood decay.
When conditions of use involve environments where the MC of the wood is higher than its saturation point (like a piece of wood in underground conditions or exposed to damp conditions, where liquid water can accumulate into the wood fibre without the possibility of drying out), wood preservatives (see Table 1) can be used to impregnate the wood fibre. The purpose of those preservatives, usually chemical products, is mainly to prevent the development of fungi and thus the decay of wood.